JPH02167427A - Grating interference type displacement gauge - Google Patents

Abstract

PURPOSE:To correct an optical axis which is bent owing to variation such as a scale surface extent defect which is small, but symmetrical in right-left relation and to eliminate its influence by constituting a diffraction grating so that a beam is reflected once by beam splitters and then mixed by a half- mirror. CONSTITUTION:The diffracted light is reflected once by using beam splitters 48A and 48B and then mixed by the half-mirror 50 by right-left symmetrical multistage type constitution. Therefore, even if a laser diode 42 varies in wavelength the directions of incident on photodetectors 22B and 22C are almost constant since the diffraction angle theta of light of 1st order is common. Namely, variation which is large, but symmetrical such as the wavelength variation of a light source, rolling on a scale, and gap variation is absorbed and the influence of the variation like this is eliminated. Further, reflected light on the surface of the scale 10 is never made incident directly on the photodetecting elements 22B and 22C.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a grating interference type displacement meter, and more particularly to a grating interference type displacement meter that can maintain stable interference even when the scale surface has poor surface roughness.

[Conventional technology]

2. Description of the Related Art Photoelectric encoders that generate periodic detection signals using a scale on which optical graduations are formed at a constant pitch have become widespread. The resolution of this photoelectric encoder is determined by the width and pitch of the optical grating and the characteristics of the electronic circuit that divides the signal after photoelectric conversion. Generally, optical gratings are manufactured by the etching method, so an optical grating of around 4 μm is close to the upper limit of final measurement accuracy, and if electronic circuits are to be used without a significant increase in cost, the final resolution is is around 1 μm, and it has been difficult to further improve the accuracy. On the other hand, as photoelectric encoders become more widespread, there is a demand for encoders that can generate detection signals with higher resolution and higher accuracy. One of the ways to improve the resolution of photoelectric encoders is to use holography technology on the scale to form graduations with a fine pitch (usually about 1 μm), and use the graduations as a diffraction grating to actively cause diffraction. A grating interferometric displacement meter that obtains a detection signal has been proposed. FIG. 7 shows a grating interference type displacement meter proposed in Japanese Patent Application Laid-Open No. 47-10034. This grating interference type displacement meter has a scale formed with a diffraction grating 10A with a pitch d,
A He-Ne tracer source 12 irradiates the diffraction grating 10A with a laser beam 14 (wavelength λ) as a luminous flux, and 0th and 1st order diffracted light (
A mixed wave of mirrors 16 and 18 that respectively reflect the rays (luminous flux), the 0th-order light of the 1st-order light reflected by the primary mirror 18, and the 1st-order light of the 0th-order light reflected by the 0th-order mirror 16. The beam splitter (coarse diffraction grating) 20 divides the beam into three parts, and light receiving elements 22A, 22B, and 22C each photoelectrically convert the mixed waves divided into three parts by the beam splitter 20. Here, each element except the scale constitutes a detector. In addition, in FIG. 7, the polarization directions of the Wei photons 24 and 26 inserted into the optical paths of the 0th-order light and the 1st-order light are set to be orthogonal to each other, and the central mixed wave divided into three The light receiving element 22A that receives the light is designed to prevent interference fringes from occurring. Therefore, in this light-receiving element 22A, a simple sum signal is obtained instead of interference fringes, so this is referred to as a reference signal Vr. In addition, just before the light receiving element 22B, an analyzer 2 for generating interference fringes is installed.
8B is arranged, and an A-phase detection signal φA based on interference fringes is generated from this light receiving element 22B. Further, a quarter-wave plate 30 and an analyzer 28C are arranged immediately before the light receiving element 22C, and from this light receiving element 220, a B phase detection signal φ having a phase difference of 90° from the A phase detection signal φA is output.
B is generated. Here, the incident angle θ of the laser beam 14 and the diffraction angle φ of the primary light are related by the following equation. d(sinθ+sinφ)=λ −−−−−−・−(1
) According to such a grating interference type displacement meter, for example, by manufacturing the diffraction grating 10A using a hologram method, 1
Since it is possible to form an optical grating with a size of 0.0 μm or less,
It is also possible to achieve a resolution of 0.01 μm.

[Problem to be achieved by the invention]

However, if the flatness of the glass surface of the scale 10 on which the diffraction grating 10A is formed is poor, in the case of a transmission type grating interference type displacement meter as shown in FIG. In the case of a defect), as shown by arrow A, there is a difference in the refraction angle and the light beam is bent, and the light receiving element 22B
The traveling directions of the two beams of light incident on , 22C are inclined to each other, and the light wavefronts perpendicular to these directions are combined into a striped pattern, making it possible to obtain uniform light interference over the entire cross-section of the beams. There wasn't. For this reason, in the past, the flatness of the scale had to be maintained at 5 μm/100 Tll or less, and if the optical axis direction was tilted due to other factors, the signal could not be detected. On the other hand, a reflection type grating interference displacement meter in which both the light source and detection system are placed on one side of the reflective scale can be installed as a separate type, since the light source and detection system can be placed on one side of the scale. Suitable for type scale. However, since diffraction of reflected light is used, the deviation of the optical path due to scale inclination or poor flatness is more severe than in the case of a transmission type, as shown in Figure 7, and it is necessary to perform installation and adjustment precisely. These tasks were difficult. The present invention has been made to solve the above-mentioned conventional problems, and reduces the influence of small but large fluctuations caused by poor flatness or inclination of the scale surface.
It is an object of the present invention to provide a grating interference type displacement meter that can obtain a stable signal and, therefore, can simplify alignment when installing a detection system.

[Means to achieve the task]

The present invention includes a scale on which a diffraction grating is formed, a light source that irradiates the diffraction grating with a light beam, and a detector that includes a light receiving element that photoelectrically converts a mixed wave of a plurality of light beams generated by the diffraction grating. In the grating interference type displacement meter that generates a detection signal that periodically changes according to the relative displacement between the scale and the detector, the plurality of light beams generated by the diffraction grating are made parallel to each other before being mixed. By providing means, the above object has been achieved. Further, the means for making the plurality of light beams parallel is a convex lens arranged so that the focus is on the refraction surface or the diffraction surface of the scale. Further, the means for making the plurality of light beams parallel is a concave mirror.

[Action and effect]

Conventionally, if there is a difference in the refraction angle (in the case of a transmission type) or the diffraction angle (in the case of a reflection type) due to poor flatness of the scale surface or the inclination of the scale, the progress of the two beams of light incident on the light receiving elements 22B and 22C will change. The directions are inclined to each other, and the light wavefronts perpendicular to the directions are combined in a striped manner, making it impossible to obtain uniform light interference over the entire overlapped cross section of the beams. For this purpose, the flatness of the scale must be maintained with high precision, and if the relative traveling direction of both beams is tilted due to other factors, the signal may not be detected, which may lead to measurement errors. Ta. As shown in FIG. 2, in the present invention, for example, by arranging a convex lens 40 so that the focus is on the refractive surface or the diffractive surface, a plurality of light beams generated by the diffraction grating are individually separated before being mixed by the half mirror 50. The flatness of the scale is 1 because the direction of travel is parallel to the direction of travel of the designer.
It is now possible to obtain a stable interference signal even at 5μIll/100μ or more. Furthermore, stable signals can now be obtained even if the optical axis is tilted due to other factors. Further, a concave mirror may be used to realize such a state. In this way, regardless of the allowable value of scale flatness or the scale inclination, the light beam after passing through the lens can always proceed in a direction parallel to the designed optical axis.
After passing through or reflecting at
Detection alignment tolerance is improved and alignment can be simplified. Furthermore, there is no need to strictly design the shape and parallelism of the light source beam. In particular, in the case of reflection-type grating interference displacement meters, which have more severe deviations in the optical path due to scale inclinations than transmission-type displacement meters and require stricter mounting and adjustment, they are easier to install and adjust, making them more effective. is even larger, and it becomes possible to realize a reflection type grating interference type displacement meter using a small light source such as a laser diode.

【Example】

Embodiments of the present invention will be described in detail below with reference to the drawings. As shown in the first embodiment of the present invention, a transmission type scale 10 on which a diffraction grating is formed, which is 1=1 with respect to the conventional example, and a collimated parallel light beam as a light source are formed. A laser diode (LD) 42 and, for example, a PIN
Light receiving elements 22A+, 22A2 consisting of photodiodes
．． 22B, 22C1 analyzer 28B, 28C1 A transmissive grating interference type displacement detector that generates a detection signal that periodically changes according to the relative displacement of the scale 10 and the detector. In the meter, a P/S splitter 44 divides the laser beam 14 from the laser diode 42 into two according to its deflection direction, and a P/S splitter 44 divides the two divided luminous fluxes into two symmetrically at an incident angle θ of the scale flatness. A pair of mirrors 46A, 46B for making the light incident on the same diffraction point C of the diffraction grating, beam splitters 48A, 48B provided to separate while reflecting only -order diffracted light, and the beam splitters 48A, 48B.
The diffracted light separated by is photoelectrically converted into a reference signal ■r=(■
The light receiving element 22Al for obtaining r, +vrb)/2
, 22A2 and a half mirror 5 for remixing the diffracted lights reflected by the beam splitters 48A and 48B.
0, the half mirror 50 and the beam splitter 48
The scale 1 is placed between A and 48B, respectively.
It is equipped with convex lenses 52A and 52B whose focal point is the zero refraction point. In this way, before the two beams diffracted by the diffraction grating are mixed by the half mirror 50, the convex lenses 52A, 52
By passing through B, the curved optical axis can be corrected by the convex lenses 52A and 52B, making it less susceptible to small but asymmetrical fluctuations such as poor flatness of the scale surface and pitching of the scale. can. In this embodiment, the diffracted light is transmitted to the beam splitter 48.
A, after reflecting once using 48B, half mirror-50
Since it has a so-called multi-stage configuration with bilateral symmetry, the diffraction angle φ of the primary light is the same even if the wavelength λ of the laser diode 42 changes, so that each light receiving element 2
The direction of incidence on 2B and 22C is almost constant. Therefore, large but symmetrical fluctuations such as wavelength fluctuations of the light source and rolling and gap fluctuations on the scale are also absorbed.
It is not affected by such fluctuations. Further, the reflected light on the surface of the scale 10 does not directly enter the light receiving element. In this embodiment, the convex lenses 52A, 52B are arranged between the beam splitters 48A, 48B and the half mirror 50, but the arrangement positions of the convex lenses 52A, 52B are not limited to this, for example, between the scale 10 and the half mirror 50. It is also possible to arrange it between beam splitters 48A, 48B. The point is that before the light flux generated by the diffraction grating is mixed, and if the focus is on the refractive surface of the scale,
Anywhere is fine. Next, a second embodiment of the present invention will be described in detail with reference to FIG. This second embodiment is a multi-stage transmission grating interference type displacement meter similar to the first embodiment, but as shown in FIG. Concave mirrors 54A and 54B are located at the
is arranged. In this second embodiment, the reference signal ■r must be obtained separately using a beam splitter (121 not shown) or the like placed somewhere else. In addition, in the first and second embodiments, the present invention has the following features:
Although the present invention has been applied to a transmission type grating interference type displacement meter including a transmission type scale 10, the scope of application of the present invention is not limited thereto, and for example, as shown in FIG. It can be similarly applied to a grating interference type displacement meter. A third and fourth embodiment of the present invention applied to a reflective grating interference displacement meter will be described below. A third embodiment of the present invention, as shown in FIG.
Convex lenses 52A and 52B are arranged between the polarizer 8B and the polarizing plates 24 and 26 so that the focal point is on the diffraction point C of the scale 60. Further, the light beam 14 from the laser diode 42 which is the light source is made to be obliquely incident on the scale 60 as shown in FIG. 5, and the light reflected by the scale 60 is
By preventing the returned light from returning to the laser diode 42, the automatic output control (APC control) of the laser diode 42 is prevented from being disturbed by the returned light. Also in this embodiment, since the optical system is bilaterally symmetrical, it is strong against fluctuations in the symmetrical optical path such as wavelength fluctuations, and can absorb the influence of wavelength fluctuations of the laser diode 42. Other points are similar to the first embodiment, so
Explanation will be omitted. Next, a fourth embodiment of the present invention will be described in detail with reference to FIG. This fourth embodiment is a reflective grating interference displacement meter similar to the third embodiment, but concave mirrors 54A and 54B are placed at the positions of beam splitters 48A and 48B instead of convex lenses 52A and 52B. . Other points are the same as those in the third embodiment or the second embodiment, so explanations will be omitted. In addition, in the above embodiments, the laser diode 42 was used as a light source, but the type of light source is not limited to this.

[Brief explanation of the drawing]

FIG. 1 is a front view showing the configuration of a first embodiment of a grating interference displacement meter according to the present invention, FIG. 2 is an optical path diagram for explaining the present invention in detail, and FIG. 3 is a diagram showing the present invention. FIG. 4 is a front view showing the configuration of the third embodiment of the present invention; FIG. 5 is an open view; FIG. 7 is a front view showing the configuration of an example of a conventional grating interference displacement meter; FIG. 8 is a front view showing the configuration of a conventional grating interference displacement meter; FIG. FIG. 0.52A, 52B... Convex lens, 2... Laser diode (LD), 8A, 48B... Beam splitter, ○... Half mirror 4A, 54B... Concave mirror.

Claims (3)

[Claims] (1) A scale on which a diffraction grating is formed, a light source that irradiates the diffraction grating with a light beam, and a detector that includes a light receiving element that photoelectrically converts a mixed wave of a plurality of light beams generated by the diffraction grating. A grating interference type displacement meter that generates a detection signal that periodically changes according to the relative displacement between the scale and the detector, wherein a plurality of light beams generated by the diffraction grating are mutually mixed before being mixed. A grating interference type displacement meter characterized by being equipped with means for parallelization. (2) The grating interference type displacement meter according to claim 1, wherein the means for collimating the plurality of light beams is a convex lens arranged so that the focus is on the refraction surface or the diffraction surface of the scale. A grating interference type displacement meter. (3) The grating interference type displacement meter according to claim 1, wherein the means for making the plurality of light beams parallel is a concave mirror.